1. Field of the Invention
The present invention relates to a communications device such as a non-contact IC card used as, for example, a ski lift pass available in a skiing area.
2. Description of the Prior Art
Referring to FIG. 11, there is illustrate a block diagram showing the structure of a prior art non-contact IC card. In the figure, reference numeral 100 denotes a host computer, 200 denotes a read/write device, and 300 denotes the non-contact IC card. The read/write device 200 included a control unit 210 for controlling the operation of the read/write device 200 according to instructions from the host computer 100, a transmitting and receiving antenna 220, and a modulating and demodulating circuit 230 for modulating a carrier signal with digital data to be transmitted to the non-contact IC card 300 and for demodulating a signal transmitted from the non-contact IC card 300. The non-contact IC card 300 is provided with a transmitting and receiving antenna 310, a demodulating circuit 320 for demodulating a signal received through the transmitting and receiving antenna 310, a modulating circuit 330 for modulating a carrier signal with digital data to be transmitted, a serial/parallel converter 340 for converting a serial signal into parallel signals and for converting parallel signals into a serial signal, a control circuit 350 for controlling the operation of the non-contact IC card 300, an EEPROM 360 in which data or the like are stored, a bus 370 through which data is transferred between the circuits, an input/output buffer 380, and a clock generating circuit 390 for generating a clock signal from the carrier signal.
Next, a description of the operation of the prior art non-contact IC card. When the read/write device 200 receives an instruction to transmit predetermined data to the IC card 300 or the like from the host computer 100, the modulating and demodulating circuit 230 modulates the carrier signal using the predetermined data to be transmitted under the control of the control unit 210. The modulated carrier signal is radiated as a radio wave by the transmitting and receiving antenna 220. When the radio wave is received by the transmitting and receiving antenna 310 of the non-contact IC card 300, it causes a high-frequency signal to be generated in the antenna. Then, the high-frequency signal is delivered to the demodulating circuit 320 and is demodulated to extract the data from the high-frequency signal. The serial/parallel converter 340 converts the demodulated serial data into corresponding parallel signals. The control circuit 350 carries out a predetermined process in accordance with these demodulated parallel signals. An example of the process is a process of transmitting data stored in the EEPROM 360 to the read/write device 200. In this case, the control circuit 350 delivers the data stored in the EEPROM 360 in the form of parallel data to the serial/parallel converter 340 by way of the input/output buffer 380. The serial/parallel converter 340 converts the parallel data input thereto into corresponding serial data and then delivers them to the modulating circuit 330. The modulating circuit 330 modulates the carrier signal with the serial digital data. The modulated carrier signal is radiated as a radio wave by the transmitting and receiving antenna 310. When the read/write device 200 receives the radio wave, the modulating and demodulating circuit 230 demodulates the radio wave to extract the data therefrom. Then, the read/write device 200 carries out a predetermined process for the data.
Referring now to FIG. 12, there is illustrated a block diagram of the structures of the serial/parallel converter 340, input/output buffer 380, and clock generating circuit 390, which are disposed within the above-mentioned non-contact IC card 300. As shown in the figure, the serial/parallel converter 340 included a baud rate generator 341 for determining the data transmission rate and a shift register 342. The input/output buffer 380 includes an output buffer 381 and an input buffer 382. The clock generating circuit 390 includes a comparator 391 and an input terminal 392. The negative input terminal of the comparator 391 is connected to a ground point. When the non-contact IC card receives a signal, the baud rate generator 341 delivers a sampling signal for sampling a serial signal from the demodulating circuit 320 for each bit of the serial data to the shift register 342. Furthermore, when a carrier signal is applied to the input terminal 392, the comparator 391 generates a clock signal having "Low" levels which correspond to the negative half waves of the carrier signal and "High" levels which correspond to the positive half waves of the carrier signal.
Referring now to FIG. 13, examples of the carrier signal which is input to the demodulating circuit 320, clock signal which is output by the comparator 391, data which are output by the demodulating circuit 320, and sampling signal which is output by the baud rate generator 341 are illustrated. As shown in the figure, the carrier signal is modulated by using a BPSK(binary phase shift keying) modulation technique so that the phase of the carrier signal is varied for each bit of digital data to be carried by the carrier according to whether the bit of the data is in the "High" level or "Low" state. In the non-contact IC card 300, the comparator 391 of the clock generating circuit 390 generates a clock signal for controlling the internal operation of the IC card from the carrier signal. The clock signal generated is delivered to the baud rate generator 341 and the frequency of a sampling signal to be generated is determined. For example, when one bit of data corresponds to a 16-clock interval, the baud rate generator 341 causes the sampling signal to transition from its "Low" level to its "High" level or from its "High" level to its "Low" level every eight clocks. The shift register 342 latches the serial data transmitted from the demodulating circuit on the rising edges of the sampling signal. When all the eight bits of the data are latched, all the bits of the data are transferred in parallel with each other to the input buffer 382. The control circuit 350 retrieves the data stored in the input buffer 382 by way of the data bus 370 when necessary.
There is a case where when the phase of the carrier signal is varied, the waveform of the carrier signal is disturbed due to a bad propagation condition of a radio wave transmitted between the read/write device 200 and the non-contact IC card 300, as shown in FIG. 13, and hence one pulse is missing from the series of clock pulses in the clock signal. If such an omission takes place successively, the rising edges of the sampling signal lag behind the carrier signal. This causes a phenomenon in which the one bit of data which is latched during this state is unstable, or the next bit is latched. Thus, the problem is that, in such a case, an error is caused in the data transmission.
Furthermore, another problem is that, when the waveform of the received carrier signal is disturbed due to noise generated in the carrier signal other than a part thereof which corresponds to the first bit of data, or multipath, the demodulating circuit 320 makes the mistake of recognizing that the phase of the carrier signal has been varied by the modulation performed by the read/write device.
Since the prior art non-contact IC card is designed as described above, it suffers from the problem that an error in the data transmission is caused by an omission of a clock pulse, which occurs during generation of the clock signal and which is caused by a change in the phase of the carrier signal due to a bad propagation condition of the radio wave, or the wrong recognition of a change in the phase of the carrier signal which is caused by disturbance of the waveform of the carrier signal due to a bad propagation condition of the radio wave.